Means for preventing the loss of charged particles injected into accelerator apparatus



March 13, 1956 w. F. WESTENDOR'P MEANS FOR PREVENTING THE LOSS OFCHARGED PARTICL v INJECTED INTO ACCELERATOR APPARATUS Filed Sept. 11,1952 2 Sheets-Sheet l Figl.

My: n in wzw on Inventor: Willem F Westendorp,

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His Attorney.

March 1956 w. F. WESTENDORP 2,738,421

MEANS FOR PREVENTING THE LOSS OF CHARGED PARTICLES INJECTED INTOACCELERATOR APPARATUS Filed Sept. 11, 1952 2 Sheets-Sheet 2 22 H22 X x,

SOURCE OF TIME mRY/NG WL77465 Inventor: Willem F We stendorp,

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' His Attorney.

United States Patent Willem F. Westendorp, Schenectady, genfiralElectric Company, a

Application September 11, 1952, Serial No. 309,007 7 Claims. (Cl.250-27) N. Y., assignor to corporation of New The present inventionrelates to charged particle accelerator apparatus and, moreparticularly, to methods and means for preventing the loss of chargedparticles injected into accelerator apparatus.

Apparatus for accelerating charged particles by means of magneticinduction effects is shown and described in my prior United StatesPatents Nos. 2,394,071, 2,394,072 and 2,394,073, all of which wereissued February 5, 1946, and assigned to the assignee of the presentinvention. Such apparatus can comprise a core of magnetic materialincluding a pair of opposed, rotationally symmetrical pole pieces whichdefine a toroidal gap wherein an evacuated annular container ispositioned. The core is excited by means of windings that are energizedby a source of timevarying voltage to produce a time-varying magneticflux which links an equilibrium orbit within the evacuated container anda time-varying magnetic guide field which traverses the equilibriumorbit. Charged particles, e. g. electrons injected along the equilibriumorbit from an electron gun positioned adjacent to the orbit within theregion of influence of the time-varying magnetic guide field, areaccelerated to high energy levels by the timevarying magnetic fluxduring a great number of revolutions while the time-varying magneticguide field constrains the patricles to follow paths along theequilibrium orbit. After acceleration to a desired energy level, thecharged particles can be directed from the equilibrium orbit to a targetfor the generation of X-rays.

As is denoted in my above-mentioned patents and more particularly inUnited States Patent No. 2,297,305, patented September 29, 1942, by D.W. Kerst and assigned to the assignee of the present invention, it isnecessary that the time-varying magnetic guide field existing betweenthe faces of the rotationally symmetrical pole pieces be spatiallydistributed to provide a field intensity essentially inverselyproportional to the radius to the power 12 in order that both radial andaxial focusing forces will be present to constrain the particles topaths along the equilibrium orbit. Specifically, this power or exponent11 must have a value lying between 0 and 1 if both radial and axialfocusing forces are to exist. The combined effect of the radial andaxial focusing forces contributed by the time-varying magnetic guidefield is to produce for a very large proportion of the charged particlesoscillatory motions along and about the equilibrium orbit, theamplitudes and frequencies of which are determined by the exponent n" ofthe time-varying magnetic guide field. In some instances, theseoscillatory motions of the charged particles cause many of them todeviate sutficiently from the equilibrium orbit after one or severalrevolutions following injection that they strike the walls of theevacuted container or return to impinge upon the rear of the injectorelectrode structure. Thus, many charged particles are lost and theultimate maximum output is severely limited.

Accordingly, a principal object of the present invention is to providemethods and means for preventing the 2,738,421 Patented Mar. 13, 1956loss of injected charged particles in charged particle acceleratorapparatus.

A further object of the present invention is to provide methods andmeans for reducing the amplitudes of the oscillatory motions of chargedparticles within magnetic induction accelerator apparatus.

A still further object of this invention is to provide methods and meansfor reducing the amplitudes of the oscillatory motions of chargedparticles without deleteriously affecting the time-varying magneticaccelerating flux in magnetic induction accelerator apparatus.

Yet another object of the present invention is to provide methods andmeans for obtaining maximum reduction of the amplitudes of theoscillatory motions of charged particles during initial periodsincluding the periods of injection of charged particles into magneticinduction accelerator apparatus.

The above objectives and others of the present invention areaccomplished, according to one but not necessarily the broadest of itsaspects, by changing the distribution of the time-varying magnetic guidefield in magnetic induction accelerator apparatus with supplementarymagnetic guide fields which can have both time-varying. andtime-constant components, the time-varying component being present onlyduring an initial period including at least the period of injection ofthe chargedparticles. By thus changing the distribution of thetimevarying magnetic guide field, the amplitudes of the oscillations ofthe charged particles are reduced and substantial loss of the chargedparticles is prevented.

The features which are desired to be protected herein are pointed outwith particularity in the appended claims. The invention itself,together with further objects and advantages thereof, can best beunderstood by reference to the following description taken in connectionwith the accompanying drawings in which:

' Fig. l is'a partly sectionalized view of accelerator apparatussuitably embodying the invention; Fig. 2 is a view taken along line 22of Fig. 1;

Figs. 3, 3A and 4 are graphical representations helpful in explainingthe invention;

Fig. 5 is a schematic circuit diagram illustrating a means of energizingthe apparatus of the invention; and

Fig. 6 is another graphical representation useful in explaining theinvention.

Referring particularly now to Figs. 1 and 2, thereis shown in exemplaryfashion magnetic induction accelerator apparatus suitably embodying theinvention. The apparatus comprises a magnetic core 1, which can belaminated to minimize the generation of eddy currents therein. Core 1includes laminated, rotationally symmetrical, opposed pole pieces 2 and3 having generallyoutwardly tapered pole faces 4 and 5 for the provisionof a magnetic guide field traversing an equilibrium orbit X, as will bemore fully described hereinafter. Coaxialv with pole pieces 2, 3 anddisposed between pole faces 4, Sis an evacuated annular container orvessel 6 of dielectric material which provides within its interior anannular chamber 7 wherein charged particles can be accelerated. Thecentral portions of pole pieces 2, 3 are terminated respectively by flatsurfaces 8, 9 between which are dis-. posed laminated metallic disks 10,11 and dielectric support spacers 12. Metallic disks 10, 11 serve thepurpose of reducing the reluctance of the magnetic path in the regionbetween surfaces 8 and 9.

Magnetic core 1 can be excited from a suitablesource of time-varyingvoltage 13 connected as indicated to series-connected energizingwindings 14, 15 surrounding pole pieces 2, 3. To minimize the currentdrawn from source 13, energizing windings 14 and 15 can be resonated bypower-factor-correcting capacitors 16. Within chamber 7 adjacentequilibrium orbit X and also within the region of influence of thetime-varying magnetic guide field existing between pole faces 4, 5during operation of the apparatus, there is provided a charged particlesource 17 which is supported from a hermetically-sealed side arm 18 ofcontainer 6. More detailed illustration and description of electron gunstructure suitable for present purposes can be found by reference to theabovementioned patents or by reference to the United States Patent No.2,484,549 of I. P. Blewett, patented October 11, 1949 and assigned tothe assignee of the present invention.

It is well understood by those familiar with magnetic inductionaccelerator apparatus that energization of windings 14, by the source oftime-varying voltage 13 results in a time-varying magnetic flux whichtraverses magnetic core 1 and pole pieces 2, 3 to provide a timevaryingmagnetic flux that links equilibrium orbit X and a time-varying magneticguide field that traverses the locus of equilibrium orbit X and thevicinity thereof between pole faces 4, 5. Electrons emitted by a gun 17at a desired timed instant near zero in the cycle of magnetic flux andfield variations are continuously accelerated during the accelerationportion of the cycle as they execute repeated revolutions along andabout equilibrium orbit X. As a consequence, the injected electrons canbe caused to assume energies of many millions of electron volts and thencan be automatically directed to impinge upon a target (not shown) toproduce X-rays for utilization exteriorly of the apparatus. Meansincluding circuits for arranging the proper timed injection andsubsequent direction of the charged particles to a target at or near theend of the acceleration cycle are disclosed in my aforementioned patentsand additionally in United States Patent No. 2,394,070 of D. W. Kerst,patented February 5, 1946, and assigned to the assignee of the presentinvention.

.As has been explained in D. W. Kerst Patent No. 2,297,305, patentedSeptember 29, 1942, and assigned to the assignee of the presentinvention, the time-varying magnetic flux linking the equilibrium orbitmay be caused to produce centripetal forces which balance the centrifugal forces upon the charged particles undergoing accelerationproviding the following relationship is satisfied:

Where A is the total change in the flux linking the equilibrium orbit, Ris the radius of the orbit and Bo is the flux density of thetime-varying magnetic guide field at the equilibrium orbit. Thecondition specified by this relationship may be realized by making thereluctance for one unit area of cross section of the magnetic path ofthe time-varying fiux greater by an appropriate amount at theequilibrium orbit than its average reluctance for one unit area of crosssection within the orbit.

The fulfillment of the foregoing condition, however, only assures stableacceleration for those charged particles which are injected tangentiallyto their instantaneous circles or orbits. The instantaneous circle ororbit is the circular orbit along which a charged particle started atthe proper position with the right energy will travel in atime-constant, radially symmetric magnetic field. With a time-varyingmagnetic flux as above specified, the loci of the instantaneous circlesof all the charged particles approach and eventually essentiallycoincide with the equilibrium orbit. Consequently, meeting the foregoingcondition does not take into consideration the requirements for stableacceleration of charged particles which tend for one reason or anotherto deviate from their respective instantaneous circles or to deviatefrom the equilibrium orbit when their respective instantaneous circlescoincide therewith. Nevertheless, by arranging the spatial distributionof the time-varying magnetic guide field 'in the vicinity of theequilibrium orbit as specified by the following relationship, bothradial Cir and axial focusing forces which tend to constrain deviatingparticles to their respective instantaneous circles or to theequilibrium orbit can be provided:

where H is the intensity of the time-varying magnetic field in thevicinity of the equilibrium orbit, H0 is the intensity of the guidefield at the equilibrium orbit, R0 is the radius of the equilibriumorbit, R is the radius of a particular point under consideration and nis an exponent having a value lying between zero and one. The outwardlydirected taper of pole faces 4 and 5 as illustrated in Fig. 1 enablesthe utilization of the conditions set forth in Equation 2.

From the foregoing it is apparent that the exponent n is a measure ofthe rate of decrease of the time-varying magnetic guide field withradius. Both radial and axial focusing forces exist if 0 n 1. For auniform field, n equals zero and no axial focusing of the particles cantake place. In a field inversely proportional to the radius, there areno radial focusing forces. Within the limits prescribed, the net radialfocusing force which is directed toward the particular instantaneouscircular orbit corresponding to the energy of an injected chargedparticle is at one unit length from this orbit:

bF B ev where F1 is the net radial focusing force above-defined, F isthe total focusing force at any particular point under consideration, eis the charge upon the particle and v is the velocity of the particle.This force corresponds to the spring constant or the restoring force ofan oscillating mass and gives rise to an oscillatory motion of eachcharged particle with respect to its respective instantaneous circularorbit. Upon injection the amplitude of these radial oscillations of aparticular charged particle is initially a distance equal to orcomparable to the distance from the injector or gun structure to theparticles instantaneous circular orbit, depending upon the directionwith which the particle leaves the injector. The stiffer the spring thefaster the oscillations; if the spring is increased in stiffness duringthe oscillations, the frequency of the oscillations increases and theamplitude decreases. Therefore, as can be seen from Equation 3, if thevalue of :n is decreased during the time of charged particle injection,the spring constant increases and the amplitude of the oscillations ofthe injected charged particle decreases.

The period of one radial oscillation Tr is expressed by where To is thetime for one passage of the particle around its instantaneous orbit. Ifn has a value of zero, Equation 4 shows that Tr equals To, and injectedparticles will strike the injector structure upon their firstrevolution. If n equals /1, the injected particles will strike the gunstructure after two revolutions. However, if 0 n there are certainvalues of n and Tr for whichthe total oscillation pattern of theinjected charged particles precesses, whereby the charged particles missthe injector structure for a relatively great number of revolutions.Nevertheless, this same precession eventually causes the points ofmaximum deviation of the charged particles to coincide with the injectorstructure or the wall of the container 6 whereby the particles are lost.Of course, some charged particles manage to avoid the injector or gunstructure and the wall of the container for a suflicient number ofrevolutions such that space charge interactions and the approach of theparticles respective instantaneous circles toward the equilibrium orbitcause the particles to be relatively closely confined to the equilibriumorbit whereupon their successful accelerationto number of the injectedcharged particles, however, the critical period is the initial periodincluding the period of injection during which the focusing forces arerelatively weak and the amplitude of the particles oscillations abouttheir instantaneous circles relatively great.

According to the present invention, the loss of charged particles due toimpingement upon the injector structure or the wall of the container isprevented by altering the exponent n of the time-varying magnetic guidefield for an initial period including the period of injection. Thus, forradial injection as is utilized with gun 17positio-ned as illustrated inFig. l, n is decreased during the build-up of the time-varying magneticguide field, whereby the amplitudes of the charged particle of theoscillations are decrease to cause the particles to avoid the injectoror gun structure even though precession of the oscillation pattern hasbrought themclose to the injector structure, as will be more fullyexplained hereinafter.

With particular reference again to Figs. 1 and 2, the desired variationin n is obtained by means of a system of auxiliary coils or windings 19,2t), 21 and 22 which can be attached with a suitable adhesive toenvelope 6. Windings 19, 29 comprise a pair of windings which can beuniformly axially spaced on opposite sides of the plane of theequilibrium orbit X; and windings 21, 22 comprise a pair or" windingswhich can be uniformly radially spaced on opposite sides of theequilibrium orbit such that they lie essentially in the plane of theorbit. It will be observed from Fig. 2 that these windings are seriallyconnected and that the direction of current flow therein when they areenergized is such that essentially no change in the flux linking theequilibrium orbit will occur as a result thereof. Thus, if the windingsare energized in a manner to be described presently such that currententers through conductor 23, the direction of current flow in winding 21will be counterclockwise, in winding 19 clockwise, in winding 22counterclockwise, and in winding 20 clockwise. Consequently, since thewindings are serially connected and the current flow through each ofthem will always be the same, they will have essentially no effect uponthe time-varying magnetic flux linking the equilibrium orbit.

The effect of current flowing in conductors 1922 upon n, or the spatialdistribution of the time-varying magnetic guide field, may best beunderstood by reference to Figs. 3, 3A and 4. Figs. 3 and 3A show theWellknown field pattern for a straight conductor in free space and areparticularly related to coil or winding 22, the cross sectionrepresented in Fig. 3 being taken on the right hand side of Fig. 1. Withthe current assumed to be fiowing in a direction into the paper, thefield H22 Varies inversely proportional to the distance along XX, a linewhich is assumed to lie essentially in the plane of the equilibriumorbit. It is apparent that with current flowing in the same direction inboth of the windings 21 and 22, the combination the fields therefromresults in cancellation near the center of container 6 or at equilibriumorbit X, an upward field to the right and a downward field to the left.The effect of windings l9 and 20 is illustrated in Fig. 4 by the vectorsH19 and H20 resulting independent- 1y from currents in windings 19 and20 respectively, the cross sections being taken also at the right handportion of Fig. 1. As will be observed, the resultant field HT isdirected upwardly at the right of the conductors and downwardly at theleft with an essentially zero field being present in the center betweenthe conductors. Again the field in the right hand portion of Fig. 1 isstrengthened on the outside beyond the equilibrium orbit and weakenedfor shorter radii, but this time by the action of windings 19' and 20.Thus, if windings 1922 are energized such that currents flow in each ofthe windings in the' directions above stated, the net field producedthereby is of such a distribution as to make the original field (seeEquation 2) more nearly uniform; hence, the

utilization of auxiliary windings 1922 decreases n in the desiredfashion. Moreover, if the windings are energized by a current pulse foran initial period including the period of injection of the chargedparticles the aforementioned loss of charged particles will beprevented.

Although the energization of windings 1922 With a pulse of currentduring an initial period including the period of injection substantiallydecreases the loss of charged particles, the decrease in n for radialinjection should be as large as possible in order to make it mosteffective. In any given accelerating apparatus, however, it is obviousthat the maximum change in n is from the value determined from the taperof the pole faces to nearly zero, where axial focusing is lost. In orderto make the rate of change of n greater, the present inventioncontemplates superimposing upon the pulsating change in n, a steadystate change which makes n larger than that for which the pole pieces ofa given accelerator apparatus are shaped. This extension of the range towhich n can be decreased is accomplished by passing a direct currentthrough the windings 19-22, whereby the total current through thewindings is a series of pulses superimposed upon a direct currentcomponent, as will be more fully explained presently. To provide thedesired steady state increase in the value of n, the direct current mustflow through the respective windings in a direction opposite to thedirection of the pulsed current flow.

The aforementioned superimposed current pulses and direct current can besupplied to windings 1922 by means of the circuit schematicallyillustrated in Fig. 5, wherein reference characters employed heretoforeare used to identify like elements. The circuit can be energized fromthe source of time-varying voltage 13 through an autotransformer 25 anda step-up transformer 26 which comprises a primary winding 27 and asecondary winding 28. During the portion of the cycle of timevaryingvoltage source 13 when charged particles are not being accelerated inthe apparatus of Fig. 1, a pulseforming network 29 which can includecapacitors 30 and inductances 31, is charged through a half-waverectifier tube 32 from the secondary winding 28 of transformer 26. At adesired time during the portion of the cycle of time-varying voltagesource 13 when charged particles are being accelerated in the apparatusof Fig. l and a relatively short time before the charged particles areinjected, a grid-controlled, gaseous discharge device 33 is renderedconductive by a suitable timed firing circuit 34, whereupon thepulse-forming network 29 is discharged through a non-inductiveresistance attenuator 35 and windings 19--22. Resistance attenuator 35includes resistors 36, 37 and 38 which, in conjunction with theinductance of windings 19-22, can be employed to control the rate ofcurrent built-up in the windings. The direction of the discharge ofpulse-forming circuit 29 through windings 1922 is such that conductor 23is positive with respect to conductor 24, whereby the current pulsesflow into the windings through conductor 23 and out through conductor24.

The direct current component is supplied in a direction opposite to thedirection of the pulsed-current flow in windings 1922 by means of asource of direct voltage 39 connected to conductor 24 through acurrent-responsive meter at an adjustable resistor 41 and a reactor 42.Resistor 41 is employed to adjust the magnitude of the direct currentcomponent detected by meter 40, and reactor 42 isolates the directcurrent circuit from the pulse circuit to prevent interference of theformer with the latter.

In Fig. 6 there is illustrated a waveform of the current throughwindings Ii -22, which can be obtained with the circuit of Fig. 5. At adesired instant, e. g., a fraction of a microsecond before the period Tiof injection of charged particles into the accelerator apparatus,discharge device .33 is rendered conductive by firing circuit 34 toinitiate a current pulse represented by curve 43. The direct currentcomponent represented by line 44 is superimposed upon the current pulsein a direction opposite that of the pulses. This has the effect ofcausing the current pulse to start from an assumed negative value asillustrated. From the foregoing explanation, it is apparent that duringthe rise of the current pulse the value of n is decreased from thevaluedetermined by pole piece design and the direct current component toa smaller value. During the relatively small time interval Ti chargedparticles are injected and accepted by the accelerator apparatus foracceleration. The amplitudes of the above-described oscillations of thecharged particles about their instantaneous circles and the equilibriumorbit are, however, decreased by the action of the current pulse,whereby they are successfully accelerated to high energy levels duringthe acceleration cycle. Atter the pulse has lasted for approximately 2microseconds, space with respect to the time-varying magnetic guidefield and,

therefore, it does not appreciably alter the value of 11 determined bypole piece design except during the desired initial period when thetime-varying magnetic guide field is relatively Weak. Nevertheless, itis within contemplation of the present invention that the direct currentcomponentcan be supplied to windings 19-22 only for the duration of thepulse by means of a conventional timing and switching circuit. The timedfiring circuit 34 can be one of several conventional circuits or, moreparticularly, may boot the saturable-strip-initiated type disclosed inmy aforementioned Patent No. 2,394,071.

Those familiar with the art to which the present invention is relatedwill readily realize that only one of the pairs of windings (-19, 20 and21, 22) can be employed, providing the resulting effect uponthetime-varying magnetic flux linking the equilibrium orbit does not aifectthe operation of the accelerator apparatus seriously. Moreover, thepresent invention is not limited to utilization with acceleratorapparatus which employs magnetic induction phenomena alone, but can alsobe used in con r junction with synchrontron apparatus such as thatdisclosed in United States Patent No. 2,485,409, patented October 18,1949, by myself and H. C. Pollock and assignedto the'assignee of thepresent invention. Further application of the present invention can bemade in con- :7

nection with non-ferromagnetic accelerating apparatus, e. g. theapparatus disclosed in United States Patent No. 2,465',7 86,patentedMarch 29,1949, by J. P. Blewett and assigned to the assignee ofthe present invention.

While this invention has been described by reference to particularembodiments thereof, alternative constructions will readily occur tothose skilled in the art. It is therefore intended in the appendedclaims to cover all such equivalent embodiments as may be within thetrue spirit-and scope of the foregoing description.

What I claim as new and desire to secure by Letters Patent'of the UnitedStates is:

1. 'ln apparatus for the acceleration of charged particles along anequilibrium orbit wherein injected. charged particles are accelerated atleast partly by the action of a time varying magnetic flux that linksthe equilibrium orbit and-are constrained to follows paths along theequilibrium orbit-by a time-varying magnetic guide'field inthe vicinityof the equilibrium orbit, the improvement which comprises-.meansjforvaryingthedistribution of thetime-var-yngm g tic gu d fiel i u e tin thet m cry n magnetic flux that links the equilibrium orbit including afirst pair of windings axially spaced on opposite sides of the plane ofthe equilibrium orbit and having essentially the same radius as theequilibrium orbit, a second pair of windings radially spaced on oppositesides of the equilbrium orbit and lying essentially in the plane of theequilibrium orbit, and means for energizing the windings during aninitial period including the period when the charged particles areinjected with time-varying currents that provide along the equilibriumorbit nearly equal but oppositely directed magnetic fields from thewindings in each of said pairs of windings, whereby substantialdeviation of the charged particles from the equilibrium orbit isprevented.

2. In apparatus for the acceleration of charged particles whereininjected charged particles are accelerated in oscillatory paths aboutand along an equilibrium orbit at least partly by the combined action ofa time-varying magnetic fiux that links the equilibrium orbit and atime-varying magnetic guide field in the vicinity of the equilibriumorbit, the improvement which comprises means for reducing the amplitudesof the oscillatory paths of the charged particles including a first pairof windings uniformly axially spaced .on opposite sides of the plane ofthe equilibrium orbit and having essentially the same radius as theequilibrium orbit, a second pair of windings radially spaced on oppositesides of the equilibrium orbit and lying essentially in the plane of theequilibrium orbit, and means for energizing said pairs of windingsduring at least an initial period including the period of injection ofthe charged particles with currents that provide along the equilibriumorbit nearly equal but oppositely directed magnetic fields from thewindings in each of said pairs of windings, whereby the distribution ofthe time-varying magnetic guide field is varied during said initialperiod and the amplitudes of the oscillatory paths of the chargedparticles are reduced.

3. In apparatus for the acceleration of charged particles whereininjected charged particles are accelerated in oscillatory paths along anequilibrium orbit at least partly by the combined action of atime-varying magnetic flux that links the equilibrium orbit and atime-varying magnetic guide field H that varies with an exponent of n inthe vicinity of the equilbrium orbit as set forth in equanon where H isthe intensity of the time varying magnetic field in the vicnity of theequilibrium orbit, Ho is the intensity of the guide field at theequilibrium. orbit, R0 is the radius of the equilibrium orbit, R is theradius of a particular point under consideration, and n is an exponenthaving a value lying between 0 and l, the improvement which comprisesmeans for reducing the amplitudes of the-oscillatory paths of thecharged particles including a first pair of windings axially spaced onopposite sides of the plane of the equilibrium orbit and havingessentially the same radius as the equilibrium orbit, a second pair ofwindings radially spaced on opposite sides of the equilibrium orbitand'lying essentially in the plane of the equilibrium orbit, and meansfor energizing said pairs of windings during at least an initial periodincluding the period of injection of the charged particles with currentsthat provide along the equilibrium orbit nearly equal but'oppositelydirected magnetic fields from the windings in each of said pairs ofwindings, whereby the exponent n is varied during said at least initialperiod without appreciably afiFecting the magnitude of the flux and theamplitudes of the oscillatory paths of the charged particles arereduced.

4. Apparatus as in claim 3 in which said means for energizing said pairsof windings comprises a pulse generatin g means connected to saidwindings.

5. Apparatus as in claim 3 in which said meansfor energizing said pairsof windingscornprises a source of direct current and pulse generatingmeans connected to rent continuously to said windings during operationof said windings to supply respectively direct current and theapparatus. pulsating current of opposite polarities to said windings.

6. Apparatus as in claim 3 in which all of said windings ReferencesCited in the file of this Patent are connected in series and saidenergizing means com- 5 UNITED STATES PATENTS prises both a source ofdirect current and a pulse generat- 2 394 070 Kerst Feb 5 1946 ing meansconnected to said windings.

7. Apparatus as in claim 6 in which said pulse generating means isoperative only during said initial period and A said source of directcurrent is connected to supply cur- 10 McMlnan 1953

